Heat exchanger systems and associated systems and methods for cooling aircraft starter/generators

ABSTRACT

Heat exchanger systems and associated systems and methods for cooling aircraft starters/generators are disclosed. A system in accordance with one embodiment includes a first fluid flow path for a first fluid, a second fluid flow path for a second fluid, and a third fluid flow path for a third fluid. The first and second flow paths are positioned proximate to the third flow path to transfer heat between the third fluid and both the first and second fluids. The third flow path is configured to allow a transfer of heat between the second and third fluids at a first transfer rate when the first fluid carries heat at a first rate, and at a second transfer rate different than the first transfer rate when the first fluid does not carry heat, or carries heat at a second rate less than the first rate. Accordingly, when the heat to be rejected by one fluid is decreased, the heat transfer rate for the remaining fluid can be increased.

TECHNICAL FIELD

The present invention is directed generally toward heat exchangersystems and associated systems and methods for cooling aircraftstarter/generators.

BACKGROUND

Existing commercial jet transport aircraft include multiple turbofanengines, each coupled to a starter/generator. The starter/generator isused both to start the turbofan engine and extract electrical power fromthe turbofan engine once the turbofan engine is started. The extractedelectrical power is then routed to electrical systems on the aircraft.

Existing starter/generators create heat that must be dissipated toprevent overheating and subsequent system failure. FIG. 1 illustrates atypical air-cooled oil cooler 20 (i.e., a heat exchanger) designed forthis purpose. The heat exchanger 20 receives hot cooling oil from thestarter/generator via a hot oil supply 31, and returns cooled oil to thestarter/generator via a cool oil return 32. Within the heat exchanger20, cold air cools the initially hot oil. The cold air can be receivedfrom a cold air supply 51, which is coupled to an air source, forexample, an air scoop located behind the fan of the turbofan engine. Awarm air return 52 returns air warmed by the oil, for example, byreintroducing the air to the fan flow, or by dumping the air overboardthe aircraft.

The foregoing arrangement has proved suitable for aircraft having asingle starter/generator associated with each turbofan engine. However,newer aircraft are placing greater electrical demands on the turbofanengines and accordingly include multiple starter/generators associatedwith each turbofan engine. One approach for cooling the additionalstarter/generators is to provide a heat exchanger generally similar tothe one shown in FIG. 1 for each starter/generator. However, this candramatically increase the weight of the heat exchanger system and istherefore not satisfactory. As a result, there is a need for a lighterweight, more efficient heat exchanger system.

SUMMARY

The present invention is directed generally toward heat exchangersystems and associated systems and methods for cooling aircraftstarter/generators. A heat exchanger system in accordance with oneaspect of the invention includes a first flow path for a first fluid, asecond flow path for a second fluid, and a third flow path for a thirdfluid. The first and second flow paths are positioned proximate to thethird flow path to transfer heat between the third fluid and both thefirst and second fluids. The third flow path is configured to allow atransfer of heat between the second and third fluids at a first transferrate when the first fluid carries heat at a first rate, and at a secondtransfer rate different than the first transfer rate when the firstfluid does not carry heat, or carries heat at a second rate less thanthe first rate.

In other aspects of the invention, the heat exchanger system can becoupled to an aircraft propulsion system having a firststarter/generator and a second starter/generator. The first flow pathcan be coupled to the first starter/generator to carry a first coolingfluid (e.g., oil), and the second flow path can be coupled to the secondstarter/generator to carry a second cooling fluid (e.g., additionaloil). The third flow path can be coupled to a cooling air intake.

In any of the foregoing arrangements, the flow paths can be integratedin such a manner that if the cooling requirements for one of the firstand second flow paths drops, the amount of heat exchanged along theother flow path can increase. For example, the first and second flowpaths can “cross” each other between their respective entrances andexits.

In a particular aspect, the first flow path can include a first entranceand a first exit, the second flow path can include a second entrance anda second exit, and the third flow path can include a third entrance anda third exit. The first entrance can be located between the thirdentrance and the second entrance, and the second exit can be locatedbetween the third entrance and the first exit. The first flow path canpass between the third entrance and the second flow path, and the secondflow path can pass between the third entrance and the first flow path.

A method for transferring heat in accordance with another aspect of theinvention includes directing a first fluid along a first flow path,directing a second fluid along a second flow path, and directing a thirdfluid along a third flow path, proximate to both the first and secondflow paths. The method can further include transferring heat between thethird fluid and the second fluid at a first transfer rate when the firstfluid carries heat at a first rate. The method can still further includetransferring heat between the third fluid and the second fluid at asecond transfer rate different than the first transfer rate when thefirst fluid does not carry heat, or carries heat at a second rate lessthan the first rate. In a further particular aspect, the shift intransfer rates can be accomplished without changing the geometricarrangements of the first, second or third flow paths.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a heat exchanger in accordancewith the prior art.

FIG. 2 is an isometric illustration of an aircraft that can house a heatexchanger system in accordance with an embodiment of the invention.

FIG. 3 is a schematic illustration of an aircraft engine coupled to twostarter/generators, and an associated heat exchanger configured inaccordance with an embodiment of the invention.

FIGS. 4A-4B illustrate heat exchangers configured to cool two fluidflows in accordance with further embodiments of the invention.

FIGS. 5A-5E illustrate heat exchangers configured to cool three fluidflows in accordance with still further embodiments of the invention.

DETAILED DESCRIPTION

The present disclosure describes heat exchanger systems and associatedsystems and methods for cooling aircraft starter/generators. In at leastsome embodiments, first and second fluid flows are cooled by a thirdfluid flow. The first and second fluid flow paths can be compactlyarranged so that each flow path has an increased exposure to the coolingcapability of the third flow when cooling requirements of the other flowpath decrease. Certain specific details are set forth in the followingdescription and in FIGS. 2-5E to provide a thorough understanding ofvarious embodiments of the invention. Well-known structures, systems,and methods often associated with these systems have not been shown ordescribed in detail to avoid unnecessarily obscuring the description ofthe various embodiments of the invention. In addition, those of ordinaryskill in the relevant art will understand that additional embodiments ofthe invention may be practiced without several of the details describedbelow.

FIG. 2 illustrates an aircraft 200 having a propulsion system 210coupled to starter/generators that are in turn cooled in accordance withan embodiment of the invention. The propulsion system 210 can includetwo turbofan engines 211. In a particular embodiment shown in FIG. 2,the engines 211 are carried by the wings 202 of the aircraft 200. Inother embodiments, the engines 211 can be carried by the fuselage 201and/or the empennage 203. The empennage 203 can also support horizontalstabilizers 204 and a vertical stabilizer 205.

FIG. 3 is a schematic illustration of one of the engines 211 describedabove with reference to FIG. 2. The engine 211 can be coupled to twostarter/generators 312 (shown as a first starter/generator 312 a and asecond starter/generator 312 b). Each starter/generator 312 can becoupled to a separate motor controller 313 (shown as a first motorcontroller 313 a and a second motor controller 313 b). Each motorcontroller 313 can be coupled to a corresponding electrical load 306(shown as a first electrical load 306 a and a second electrical load 306b). Each electrical load 306 can include electric actuators, flight deckcomputers and displays, fans, motors, and/or other electrically poweredaircraft equipment.

Both the first starter/generator 312 a and the second starter/generator312 b can be coupled to a common heat exchanger 320 for cooling.Accordingly, a first supply/return path 330 can conduct a cooling fluid(e.g., an oil) between the first starter/generator 312 a and the heatexchanger 320. A second supply/return path 340 can conduct a secondfluid (e.g., an independent supply of oil) between the secondstarter/generator 312 b and the heat exchanger 320. A thirdsupply/return path 350 can direct a third fluid to the heat exchanger320 to cool the first and second fluids. In one aspect of thisembodiment, the third fluid can include air removed from the fan flow ofthe engine 211, and returned to the fan flow after passing through theheat exchanger 320. In other embodiments, the third fluid can beextracted from and/or returned to other portions of the aircraft, and/orcan include fluids other than air (e.g., aviation fuel).

FIG. 4A is a schematic, isometric illustration of the heat exchanger 320shown in FIG. 3. The heat exchanger 320 can be coupled to a first fluidsupply 431 at a first fluid entrance 433, and to a first fluid return432 at a first fluid exit 434. A first flow path 435 directs the firstfluid from the first fluid entrance 433 to the first fluid exit 434. Thefirst flow path 435 can include a conduit 421 having multiple coolingfins 422 (only a few of which are shown in FIG. 4A) to cool the firstfluid by exposure to the third fluid.

The heat exchanger 320 can receive the second fluid via a second fluidsupply 441 at a second fluid entrance 443 a, and can return the secondfluid via a second fluid return 442 at a second fluid exit 444 a. Asecond flow path 445 directs the second fluid from the second fluidentrance 443 a to the second fluid exit 444 a. Like the first flow path435, the second flow path 445 can include a conduit 421 having multipleexternal cooling fins 422.

The first and second fluids are cooled by the third fluid in across-flow heat exchanger arrangement. The third fluid is received via athird fluid supply 451 at a third fluid entrance 453. The third fluidcan be discharged from the heat exchanger 320 to a third fluid return452 at a third fluid exit 454. A third flow path 455 directs the thirdfluid proximate to the first flow path 435 and the second flow path 445to cool the first and second fluids, respectively. The third flow path455 can include vanes 423 or other structures positioned to direct thethird fluid in a manner that efficiently cools both the first and secondfluids.

One feature of an embodiment of the heat exchanger 320 shown in FIG. 4Ais that the first flow path 435 and the second flow path 445 cross overeach other in a region where they are exposed to the third fluid. In aparticular aspect of an embodiment shown in FIG. 4A, the first flow path435 and the second flow path 445 cross over each other only once, but inother embodiments, the two flow paths can cross over each other morethan once. An aspect of any of these embodiments is that the first flowpath 435 passes close to the third fluid entrance 453 over at least partof its length, and the second flow path 445 passes close to the thirdfluid entrance 453 over at least part of its length. Put another way,the third flow path 455 can include a first segment 455 a and agenerally parallel second segment 455 b. The first segment 455 a passes(sequentially) adjacent to the first fluid entrance 433 and then thesecond fluid entrance 443 a. The second segment 455 b passes(sequentially) adjacent to the second fluid exit 444 a and the firstfluid exit 434.

One advantage of the foregoing arrangement is that the third fluid canprovide roughly equal cooling benefits for both the first and secondfluids. For example, each of the first and second flow paths 435, 445can include a segment closer to the third fluid entrance 453 (which isrelatively cool) than to the third fluid exit 454 (which is warmer), andanother segment that is closer to the third fluid exit 454 than to thethird fluid entrance 453. Accordingly, each of the first and secondfluids can have roughly equal exposure to relatively cool portions ofthe third fluid, and warmer portions of the third fluid.

Another advantage is that, when the heat transfer requirements for oneof the first and second fluids is reduced, the rate of heat transferfrom the remaining fluid can be increased. For example, in one mode ofoperation (e.g., “normal” operation), the rate at which heat istransferred away from the first fluid is relatively high toward thefirst fluid entrance 443 and, because the second flow path 445 ispositioned between the first fluid exit 434 and the third fluid entrance453, the rate at which heat is transferred away from the first fluid issomewhat lower toward the first fluid exit 434. Conversely, the rate atwhich heat is transferred from the second fluid may be relatively lowtoward the second fluid entrance 443 a because the first flow path 435is positioned between the second fluid entrance 443 a and the thirdfluid entrance 453. The rate at which heat is transferred away from thesecond fluid can be increased toward the second fluid exit 444 a becausethe first flow path 435 is not interposed between the second flow path445 and the third fluid entrance 453 in this region. If, in a secondmode of operation, the rate at which heat is carried by the second fluiddecreases (e.g., because the fluid flow rate decreases or because thetemperature of the second fluid decreases), then the temperature of thethird fluid after passing over the second flow path 445 proximate to thesecond fluid exit 444 a will increase. As a result, the third fluidpassing over the first flow path 435 proximate to the first fluid exit434 will cool the first fluid at a greater rate. Therefore, the overallrate at which heat is transferred away from the first fluid will tend toincrease as the rate at which heat carried by the second fluiddecreases. In the limit, when the flow rate of the second fluid isreduced to zero, the entire capacity of the heat exchanger 320 can bedirected to transferring heat away from the first fluid alone.

The foregoing arrangement can be advantageous for several reasons. Oneis that the heat transfer requirements for both the first and secondfluids can be met by a single device, which can reduce duplicativestructures and can accordingly reduce the overall weight of the heatexchanger 320 when compared to two separate heat exchangers, eachdedicated to cooling one of the first and second fluids. This advantagecan be particularly useful in aircraft installations, where low weighthas a high priority.

Another advantage of the foregoing arrangement is best understood withreference to FIG. 3. If, for any reason, the second starter/generator312 b has a reduced cooling requirement, and therefore discharges heatto the heat exchanger 320 at a lower rate, the rate at which heat istransferred away from the first starter/generator 312 a can beincreased. Conversely, if the heat transfer requirements of the firststarter/generator 312 a are reduced, the rate at which heat istransferred away from the second starter/generator 312 b can beincreased. This result is not attainable with two independent,stand-alone heat exchangers.

The heat transfer requirements for either the first or secondstarter/generator can drop as a result of one or more of severalcircumstances. For example, the first or second electrical loads 306 a,306 b can be different. One of the motor controllers 313 can fail, orone of the starter generators 312 can fail. In at least some of thesecases, if the heat transfer requirements for one of thestarter/generators 312 falls (for example, due to an equipment failure),at least some of the electrical load coupled to that starter/generator312 can be shifted to the remaining (operating) starter/generator 312.As a result, the remaining starter/generator 312 will have an increasedheat transfer requirement. As discussed above, this increased heattransfer requirement can be met by the heat exchanger 320 because theheat exchanger 320 has an effectively increased cooling capacity as aresult of the heat transfer requirements for the failedstarter/generator being reduced. This in turn can allow the (operating)starter/generator 312 to operate in an overload mode for a greaterperiod of time than it would with an existing heat exchangerarrangement.

Yet another feature of embodiments described above is that the shift inrelative heat transfer rates for the first and second fluids can beaccomplished without the need for moving parts (e.g., movable vanes,valves or other mechanical devices). In other words, the geometricarrangements and structural configurations of the first, second andthird flow paths, respectively, can remain generally the same even asthe heat transfer rates shift. An advantage of this feature is that theheat exchanger 320 can be relatively simple to manufacture, maintain andoperate.

FIGS. 4B-5E illustrate heat exchangers having other arrangements thatalso include many of the features described above. For example, FIG. 4Billustrates a heat exchanger 420 having a geometry generally similar tothat of the heat exchanger 320 described above with reference to FIG.4A, but with the positions of the second fluid entrance and the secondfluid exit reversed. Accordingly, the heat exchanger 420 can include asecond fluid entrance 443b positioned close to the third fluid entrance453, and a second fluid exit 444b positioned close to the third fluidexit 454. Whether a designer chooses a configuration generally similarto that of FIG. 4A or 4B (or any of FIGS. 5A-5E) depends on aspects thatinclude the details of a particular installation.

FIGS. 5A-5E schematically illustrate side views of heat exchangers thatinclude a fourth flow path for cooling a fourth fluid, in addition toflow paths for cooling the first and second fluids. In an aircraftinstallation, the fourth fluid can in turn be used to cool a componentin addition to the starter/generators 312 (FIG. 3) without addinganother heat exchanger. For example, referring first to FIG. 5A, a heatexchanger 520 a can include a first flow path 535 that crosses a secondflow path 545. The first flow path 535 is coupled between a first fluidentrance 533 and a first fluid exit 534, and the second flow path 545 iscoupled between a second fluid entrance 543 and a second fluid exit 544.The heat exchanger 520a can also include a fourth flow path 565 aextending between a fourth fluid entrance 563 a and a fourth fluid exit564 a. The third fluid passes along a third flow path 555 between athird entrance 553 and a third fluid exit 554 to cool the first, secondand fourth fluids. The fourth flow path 565 a can be positioned upstream(with respect to the third flow path 555) of the first flow path 535 andthe second flow path 545 and can accordingly provide “preferential”cooling for the fourth fluid. This arrangement may be suitable when thecooling requirements for the fourth fluid are expected to be greaterthan the cooling requirements for the first and second fluids.

Referring now to FIG. 5B, a heat exchanger 520b can have a configurationgenerally similar to that described above with reference to FIG. 5A, butwith the fourth flow path 565 b located downstream of the first andsecond flow paths 535, 545. The fourth flow path 565 b accordinglyreceives the fourth fluid via a fourth fluid entrance 563 b and exitsthe fourth fluid via a fourth fluid exit 564 b, both of which arelocated downstream of the first and second flow paths 535, 545 (relativeto the flow direction of the third fluid). This arrangement may besuitable for situations in which the cooling requirements for the fourthfluid are expected to be less than the cooling requirements for thefirst and second fluids.

FIGS. 5C and 5D illustrate heat exchangers 520 c and 520 d,respectively, having fourth flow paths that are in parallel with thefirst and second flow paths. These arrangements may be suitable forsituations in which the cooling requirements for the first, second andfourth fluids are roughly equal. For example, referring first to FIG.5C, the heat exchanger 520 c can include a fourth flow path 565 ccoupled between a fourth fluid entrance 563 c and a fourth fluid exit564 c. The fourth flow path 565 c is arranged in parallel with the firstflow path 535 and the second flow path 545 so that changes in the heattransfer rate from the fourth fluid have a reduced or nonexistent effecton the heat transfer rate from the first and second fluids. Inparticular, the fourth flow path 565 c can be offset into the plane ofFIG. 5C, relative to the first flow path 535 and the second flow path545.

FIG. 5D illustrates a heat exchanger 520 d having a fourth flow path 565d (with a fluid entrance 563 d and fluid exit 564 d) that is alsoarranged in parallel with the first and second flow paths 535, 545. Asdescribed above with reference to FIG. 5C, the heat transfer rate fromthe fourth fluid can be generally independent of the heat transfer ratefrom the first and second fluids.

FIG. 5E illustrates a heat exchanger 520 e for which flow paths for eachof the first, second and fourth fluids cross each other. Accordingly, ifthe heat transfer requirements for any one of the flow paths is reduced,the other two flow paths can realize an increased heat transfer rate. Ina particular aspect of the embodiment shown in FIG. 5E, a first fluidflow path 535 e extends between a first fluid entrance 533 e and a firstfluid exit 534 e and crosses both a second fluid flow path 545 e and afourth fluid flow path 565 e. The second fluid flow path 545 e extendsbetween a second fluid entrance 544 e and a second fluid exit 543 e, andthe fourth fluid flow path 565 e extends between a fourth fluid entrance563 e and a fourth fluid exit 5644 e. The third fluid passes adjacentthe first, second and fourth fluid flow paths between the third fluidentrance 553 and the third fluid exit 554. Because the first, second andfourth fluid flow paths cross each other, if the rate at which any ofthe flows carries heat energy falls, the effective heat transfer ratefor both of the remaining two flow paths can be increased.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thespirit and scope of the invention. For example, many of the heatexchangers described above have been described in the context of heatexchangers for aircraft engine starter/generators. In other embodiments,heat exchangers having generally similar characteristics can beintegrated with other stationary or mobile devices. In many of theembodiments described above, heat is described as being transferred to athird fluid from a first, second (and optionally, fourth fluid or stillfurther fluids). In other embodiments, the direction of heat transfercan have the opposite sense (e.g. to the third flow). In still a furtherexample, many of the embodiments described above are configured toproduce a greater overall heat transfer rate for one fluid when the rateat which heat energy carried by another fluid is reduced, without theneed for moving parts. In other embodiments, the heat exchanger caninclude moving parts in addition to or in lieu of the flow patharrangements described above.

Aspects of the invention described in the context of particularembodiments may be combined or eliminated in other embodiments. Forexample, the conduits and fins described in the context of FIGS. 4A-4Bcan be included in the arrangements shown in FIGS. 5A-5E. Althoughadvantages associated with certain embodiments of the invention havebeen described in the context of those embodiments, other embodimentsmay also exhibit other such advantages. Additionally, none of theforegoing embodiments need necessarily exhibit such advantages to fallwithin the scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. An aircraft, comprising: a fuselage; a wing; a propulsion systemcoupled to at least one of the wing and the fuselage; a firststarter/generator coupled to the propulsion system; a secondstarter/generator coupled to the propulsion system; and a heat exchangercoupled to the first and second starter/generators, the heat exchangerincluding: a first flow path coupled to the first starter/generator tocarry a first cooling fluid; a second flow path coupled to the secondstarter/generator to carry a second cooling fluid; and a third flow pathfor a third cooling fluid; wherein the first and second flow paths arepositioned proximate to the third flow path to transfer heat between thethird cooling fluid and both the first and second cooling fluids, andwherein the third flow path is configured to allow a transfer of heatbetween the second and third cooling fluids at a first transfer ratewhen the first cooling fluid carries heat at a first rate, and at asecond transfer rate different than the first transfer rate when thefirst cooling fluid does not carry heat or carries heat at a second rateless than the first rate.
 2. The aircraft of claim 1 wherein thepropulsion system includes a turbofan engine, and wherein eachstarter/generator is coupled to the turbofan engine.
 3. The aircraft ofclaim 1 wherein each of the first and second starter/generators iscoupled to a different electrical load so that the cooling requirementsof the first starter/generator are different than the coolingrequirements of the second starter/generator over at least a portion ofthe time during which the starter/generators are operational.
 4. Theaircraft of claim 1 wherein the third flow path includes an entrance andan exit, and wherein the first flow path passes between the entrance andthe second flow path, and the second flow path passes between theentrance and the first flow path.
 5. The aircraft of claim 1 wherein thefirst flow path includes a first entrance and a first exit, the secondflow path includes a second entrance and a second exit, and the thirdflow path includes a third entrance and a third exit, and wherein thefirst entrance is located between the third entrance and the secondentrance, the second exit is located between the third entrance and thefirst exit, the first flow path passes between the third entrance andthe second flow path, and the second flow path passes between the thirdentrance and the first flow path.
 6. The aircraft of claim 1 wherein thegeometric arrangements of first, second and third flow paths,respectively, are at least generally the same whether the transfer ofheat is at the first transfer rate or the second transfer rate.
 7. Aheat exchanger system, comprising: a first flow path for a first fluid;a second flow path for a second fluid; a third flow path for a thirdfluid; and wherein the first and second flow paths are positionedproximate to the third flow path to transfer heat between the thirdfluid and both the first and second fluids, and wherein the third flowpath is configured to allow a transfer of heat between the second andthird fluids at a first transfer rate when the first fluid carries heatat a first rate, and at a second transfer rate different than the firsttransfer rate when the first fluid does not carry heat or carries heatat a second rate less than the first rate.
 8. The system of claim 7,further comprising: a first aircraft engine starter/generator coupled tothe first flow path, wherein the first fluid includes a liquid coolantfor the first aircraft starter/generator; a second aircraft enginestarter/generator coupled to the second flow path, wherein the secondfluid includes a liquid coolant for the second aircraftstarter/generator; an air intake coupled to the third flow path, whereinthe third fluid includes air for cooling the first and second fluids;and wherein the third flow path includes an entrance and an exit, withthe first flow path passing between the entrance and the second flowpath, and the second flow path passing between the entrance and thefirst flow path, further wherein the third fluid cools the second fluidat a second rate higher than the first rate when the first fluid doesnot carry heat or carries heat at a second rate less than the firstrate, and wherein the first, second and third flow paths, respectively,have at least generally the same structural configurations whether thethird fluid cools at the first rate or the second rate.
 9. The system ofclaim 7 wherein the first flow path is coupled to a first aircraftengine starter/generator, and wherein the second flow path is coupled toa second aircraft engine starter/generator, and wherein the third flowpath is coupled to an intake for cooling air.
 10. The system of claim 7wherein the third flow path includes an entrance and an exit, andwherein the first flow path passes between the entrance and the secondflow path, and the second flow path passes between the entrance and thefirst flow path.
 11. The system of claim 7 wherein the first flow pathincludes a first entrance and a first exit, the second flow pathincludes a second entrance and a second exit, and the third flow pathincludes a third entrance and a third exit, and wherein the firstentrance is located between the third entrance and the second entrance,the second exit is located between the third entrance and the firstexit, the first flow path passes between the third entrance and thesecond flow path, and the second flow path passes between the thirdentrance and the first flow path.
 12. The system of claim 7 wherein thefirst flow path includes a first entrance and a first exit, the secondflow path includes a second entrance and a second exit, and the thirdflow path includes a third entrance and a third exit, and wherein thefirst entrance is located between the third entrance and the secondexit, the second entrance is located between the third entrance and thefirst exit, the first flow path passes between the third entrance andthe second flow path, and the second flow path passes between the thirdentrance and the first flow path.
 13. The system of claim 7, furthercomprising an aircraft that houses the first, second and third flowpaths.
 14. The system of claim 7 wherein the geometric arrangements offirst, second and third flow paths, respectively, are at least generallythe same whether the transfer of heat is at the first transfer rate orthe second transfer rate.
 15. The system of claim 7, further comprisinga fourth flow path for a fourth fluid, the fourth flow path being atleast proximate to the third flow path to transfer heat between thethird and fourth flow paths.
 16. The system of claim 7 wherein the firstflow path includes a first entrance and a first exit, the second flowpath includes a second entrance and a second exit, and the third flowpath includes a third entrance and a third exit, and wherein the thirdflow path includes first and second generally parallel segmentspositioned between the third entrance and the third exit, the firstsegment passing sequentially adjacent to the first flow path and thesecond flow path, the second segment passing sequentially adjacent thesecond flow path and the first flow path.
 17. The system of claim 7wherein the first flow path includes a first entrance and a first exit,the second flow path includes a second entrance and a second exit, andthe third flow path includes a third entrance and a third exit, andwherein the third flow path includes first and second generally parallelsegments positioned between the third entrance and the third exit, thefirst segment passing sequentially adjacent to the first flow path atthe first entrance and the second flow path at the second exit, thesecond segment passing sequentially adjacent the second flow path at thesecond entrance and the first flow path at the first exit.
 18. A methodfor transferring heat, comprising: directing a first fluid along a firstflow path; directing a second fluid along a second flow path; directinga third fluid along a third flow path proximate to both the first andsecond flow paths; transferring heat between the third fluid and thesecond fluid at a first transfer rate when the first fluid carries heatat a first rate; and transferring heat between the third fluid and thesecond fluid at a second transfer rate different than the first transferrate when the first fluid does not carry heat or carries heat at asecond rate less than the first rate.
 19. The method of claim 18 whereintransferring heat at a second transfer rate includes transferring heatat a second transfer rate while the geometric arrangements of the first,second and third flow paths, respectively, are at least generally thesame as when transferring heat at the first transfer rate.
 20. Themethod of claim 18, further comprising: directing the first fluid from afirst aircraft engine starter/generator to a heat exchanger, wherein thefirst fluid includes a first cooling fluid; directing the second fluidfrom a second aircraft engine starter/generator to the heat exchanger,wherein the second fluid includes a second cooling fluid; directing thethird fluid to the heat exchanger, wherein the third fluid includes airfor cooling the first and second fluids; and wherein directing the thirdfluid along the third flow path includes directing the third fluid froman entrance of the heat exchanger to an exit of the heat exchanger, withthe first flow path passing between the entrance and the second flowpath, and the second flow path passing between the entrance and thefirst flow path.
 21. The method of claim 18, further comprising:directing the first fluid from a first aircraft engine starter/generatorto a heat exchanger, wherein the first fluid includes a first coolingfluid; directing the second fluid from a second aircraft enginestarter/generator to the heat exchanger, wherein the second fluidincludes a second cooling fluid; and directing the third fluid to theheat exchanger, wherein the third fluid includes air for cooling thefirst and second fluids.
 22. The method of claim 18 wherein: the firstflow path includes a first entrance and a first exit; the second flowpath includes a second entrance and a second exit; the third flow pathincludes a third entrance and a third exit; passing the first fluidincludes passing the first fluid along a first flow path that extendsaway from the third fluid entrance; and passing the second fluidincludes passing the second fluid along a second flow path that extendstoward the third fluid entrance.
 23. The method of claim 18 wherein: thefirst flow path includes a first entrance and a first exit; the secondflow path includes a second entrance and a second exit; the third flowpath includes a third entrance and a third exit, with the first fluidentrance located between the third fluid entrance and the second fluidexit; passing the first fluid includes passing the first fluid along afirst flow path that extends away from the third fluid entrance; andpassing the second fluid includes passing the second fluid along asecond flow path that extends away from the third fluid entrance. 24.The method of claim 18 wherein: the first flow path includes a firstentrance and a first exit; the second flow path includes a secondentrance and a second exit; the third flow path includes a thirdentrance and a third exit; and passing the third fluid includes passingthe third fluid along first and second generally parallel segments ofthe third flow path positioned between the third entrance and the thirdexit, with the third fluid in the first segment passing sequentiallyadjacent to the first flow path and the second flow path, and with fluidin the second segment passing sequentially adjacent the second flow pathand the first flow path.
 25. The method of claim 18 wherein the first,second and third flow paths are located in a single heat exchanger, andwherein the method further comprises directing a fourth fluid along afourth flow path in the heat exchanger, and transferring heat betweenthe third fluid and the fourth fluid.